KEY SECTOR 03. WATER AND CLIMATE CHANGE IN MONTANA Wyatt F. Cross, John LaFave, Alex Leone, Whitney Lonsdale, Alisa Royem, Tom Patton, and Stephanie McGinnis Water is the lifeblood of Montana. We depend on an adequate supply of clean water for nearly every aspect of our lives, including food production, hydroelectric power, domestic and industrial uses, and sustaining our treasured natural ecosystems. Water is also strongly influenced by climate, as changes in temperature and precipitation consistently alter patterns of water availability and quality throughout the state. It is thus critical that we understand the impacts of climate change on Montana’s water resources. This chapter synthesizes scientific information on how climate change is influencing the supply and distribution of water in Montana. The information presented here represents an essential first step— understanding what’s changing—within the longer-term, iterative process of adapting and improving our resilience to the challenges of an uncertain climate future. 2017 MONTANA CLIMATE ASSESSMENT | 71 KEY MESSAGES • Montana’s snowpack has declined over • Local responses of groundwater resources the observational record (i.e., since the to climate change will depend on whether 1930s) in mountains west and east of the aquifers are directly sensitive to climate Continental Divide; this decline has been variability, are buffered from climate by most pronounced since the 1980s. [high water-use practices such as irrigation, or are agreement, medium evidence] 11 used to meet water demands that exceed or replace surface water supplies. [high • Warming temperatures over the next century, agreement, robust evidence] especially during spring, are likely to reduce snowpack at mid and low elevations. [high • Groundwater demand will likely increase agreement, robust evidence] as elevated temperatures and changing seasonal availability of traditional surface- • Historical observations show a shift toward water sources (e.g., dry stock water ponds earlier snowmelt and an earlier peak in or inability of canal systems to deliver water spring runoff in the Mountain West (including in a timely manner) force water users to Montana). Projections suggest these patterns seek alternatives. [high agreement, medium are very likely to continue into the future evidence] as temperatures increase. [high agreement, robust evidence] • Multi-year and decadal-scale droughts have been, and will continue to be, a • Earlier onset of snowmelt and spring runoff natural feature of Montana’s climate will reduce late-summer water availability [high agreement, robust evidence]; rising in snowmelt-dominated watersheds. [high temperatures will likely exacerbate drought agreement, robust evidence] when and where it occurs. [high agreement, medium evidence] • Long-term (decadal and multi-decadal) variation in total annual streamflow is largely • Changes in snowpack and runoff timing will influenced by patterns of climate variability; likely increase the frequency and duration of the influence of climate warming on these drought during late summer and early fall. patterns is uncertain. [high agreement, [high agreement, medium evidence] medium evidence] • A warming climate will strongly influence • Total annual streamflows are projected to Montana’s snowpack, streamflow dynamics, increase slightly for most Montana rivers, but and groundwater resources, with far-reaching the magnitude of change across the state consequences for social and ecological and agreement among models vary. [medium systems. [high agreement, medium evidence] agreement, medium evidence] 11 A reminder that throughout the MCA we assess our confidence in the key messages by considering a) the level of agreement among experts with relevant knowledge, and b) the quality of the evidence. We use these two factors and the criteria described in the National Climate Assessment to assign the confidence ratings expressed in this chapter. See sidebar titled “Expressed Confidence in MCA Key WATERMessages” in the Introduction chapter. INTRODUCTION Our discussion focuses on climate as a principal driver of change for water resources. However, it is important to note that there are many additional drivers beyond climate, such as population growth and associated changes in land use, that strongly influence our demand for water both now and into the future. Indeed, much of Montana’s water is already fully allocated to various uses (Table 3-1) (Arnell 1999; VörÖsmarty et al. 2000; Montana Department of Natural Resources and Conservation [MT DNRC] 2015), suggesting that creative and collaborative water management strategies will be essential for sustaining abundant and clean water into the future (see Missouri River sidebar). Table 3-1. Water use in Montana from the Montana State Water Plan (MT DNRC 2015). Water use can be non-consumptive (e.g., hydropower where water returns to the surface water system), partially consumptive (e.g., irrigation where some water returns to the system), or consumptive (e.g., reservoir evaporation where water is non-recoverable with respect to continued surface water use). See the DNRC Regional Basin Plans (MT DNRC 2014a, b, c, d) for additional local detail. Also note that water used for hydropower is often counted multiple times as it travels through a series of power-generating plants. Water usage Annual acre-feet (m3) % of category Total Water Use Hydropower 72,000,000 (8.9x1010) 85.9 Irrigation diversion 10,395,000 (1.3x1010) 12.4 Reservoir evaporation 1,002,000 (1.2x109) 1.2 Municipal, stock, industrial, and domestic use 384,000 (4.7x108) 0.5 Consumptive Water Uses Agricultural irrigation 2,414,000 (3.0x109) 67.3 Reservoir evaporation 1,002,000 (1.2x109) 28.0 Municipal 72,000 (8.9x107) 2.0 Stock water 42,500 (5.2x107) 1.2 Thermoelectric 27,400 (3.4x107) 0.08 Domestic 13,900 (1.7x107) 0.4 Industrial 10,400 (1.3x107) 0.03 WATER 2017 MONTANA CLIMATE ASSESSMENT | 73 Basin Study of the Missouri River Watershed The current Montana Climate Assessment is focused on understanding relationships between climate change and water resources, with minimal focus on how water use and water management interact with climate. To help advance this important knowledge gap, the Montana Department of Natural Resources and Conservation is partnering with the US Bureau of Reclamation (Reclamation) to conduct a Basin Study of the Missouri River watershed from the headwaters to Fort Peck Reservoir, including the Musselshell River basin (USBR 2014b). Purpose.—The purpose of the Basin Study is to understand potential future changes in basin water supplies and demands, and to analyze possible adaptation strategies for providing water needs into the future. The study builds on Reclamation’s Upper Missouri Impact Assessment (USBR forthcoming) and the Montana State Water Plan (MT DNRC 2015), which evaluate how existing infrastructure would perform under anticipated future conditions. Modeling.—As part of the study, climate and hydrology models will be used to project future water supplies and demands for the Missouri River and its major tributaries. The output from these models will serve as input data to a river-system management model that simulates streamflows, water diversions, water use, return flows, and reservoir operations. Reservoirs simulated in the model include Clark Canyon, Canyon Ferry, Gibson, and Tiber reservoirs, as well as some smaller state and private projects. Desired results.—Output from the river system model is being used to identify likely imbalances in water supply and demand as compared to past and existing operations under known climate and hydrologic conditions. 74 | WATER AND CLIMATE CHANGE Model output will be used to evaluate adaptation and mitigation strategies including reservoir operational changes, modification of existing facilities, and improved water management. Public participation is a key element of the Basin Study, especially for identifying and developing adaptation strategies. Text and figure contributed by Larry Dolan (MT DNRC) and Marketa McGuire (US Bureau of Reclamation). Climate change and the water cycle The effects of climate change on Montana’s water resources can be best understood by starting with a brief description of the water cycle (Figure 3-1). The water cycle refers to the continuous movement of water from the atmosphere to the Earth’s surface and back, shifting between gaseous (water vapor), liquid, and solid (snow or ice) phases. Each of these phases of the water cycle can be impacted by climate change. 2017 MONTANA CLIMATE ASSESSMENT | 75 Figure 3-1. Simplified schematic of the water cycle. Artwork by Jenny McCarty. The primary atmospheric source for the water cycle is evaporated water from the ocean. In Montana, much of the winter snowfall that accumulates in the mountains melts in the spring to produce streamflow and recharge groundwater aquifers. This same water supports municipalities and industry throughout the year and is used to irrigate crops in the summer. Some irrigation water will directly support plant growth and some will trickle back into groundwater aquifers. Much of this same water will return to the atmosphere as water vapor through evaporation or plant transpiration, thus completing the water cycle. Precipitation as rainfall is a significant part of the water cycle in Montana, and its contribution to runoff can exceed that of snowfall in prairie environments in the state. Changes in temperature near the Earth’s surface will have large effects on how water enters Montana (e.g.,